Silicon-on-insulator transistors having improved current characteristics and reduced electrostatic discharge susceptibility

ABSTRACT

An SOI MOSFET having improved electrical characteristics includes a low barrier body contact under the source region, and alternatively under the drain region, to facilitate collection and removal of current carriers generated by impact ionization. Fully-depleted and non-fully-depleted SOI MOSFETs can be integrated on the same chip by providing some transistors with thicker source and drain regions with a recessed channel therebetween and by selective channel dopant implant. Accordingly, digital circuitry and analog circuitry can be combined on one substrate. Improved electrostatic discharge protection is provided by fabricating transistors for the protection circuit directly in the supporting substrate by first removing the silicon thin film and underlying insulation barrier. Alternatively, improved transistors for electrostatic discharge protection can be formed in the silicon film by fabricating the transistor in a plurality of electrically isolated segments, each segment having source and drain regions separated by a channel region with the regions being electrically interconnected with like regions in other segments. Increased ESD current can be realized as compared to the ESD current for a wider unsegmented device.

This invention was made with Government support under Contract No.F49620-93-C-0014 awarded by the Air Force Office of ScientificResearch/Joint Services Electronics Program. The government has certainrights to this invention.

This patent application is a division of divisional application Ser. No.08/461,355 filed Jun. 5, 1995, now U.S. Pat. No. 5,982,003, which is adivision of application Ser. No. 08/224,363 filed Apr. 7, 1994, now U.S.Pat. No. 5,489,792.

BACKGROUND OF THE INVENTION

This invention relates generally to semiconductor devices, and moreparticular the invention relates to field effect devices fabricated insilicon-on-insulator structures.

Silicon-on-insulator (SOI) technology employs a layer of semiconductormaterial overlying an insulation layer on a supporting bulk wafer. Thestructure can be formed by a number of well-known techniques, such aszone melting and recrystallization (ZMR) separation by implanted oxygen(SIMOX) or Bonded and Etchback (BESOI). Typically, the structurecomprises a film of monocrystalline silicon on a buried layer of siliconoxide in a monocrystalline silicon substrate.

Field effect transistors such as MOSFETs fabricated in the silicon filmof an SOI structure have many advantages over MOSFETs fabricated on thetraditional bulk silicon substrates including resistance toshort-channel effect, steeper subthreshold slopes, increased currentdrive, higher packing density, reduced parasitic capacitance, andsimpler processing steps. In the past, the range of SOI applications hasbeen limited due to high cost and inferior crystalline quality of SOIwafers. However, recent advancements in the SOI silicon film quality,buried oxide quality, and manufacturing throughput have opened the doorto a multitude of ultra large scale integration (ULSI) applications.Combined with the continually increasing cost of bulk silicon submicronintegrated circuit processes and the lower complexity/cost of SOIintegrated circuit processes, SOI technology shows great potential tobecome the low cost mainstream production technology.

Despite all of the attractiveness of SOI technology, there are obstacleswhich cancel part of the benefit of using SOI for high-performance,high-density ULSI circuits. The problem is especially severe for analogcircuits or mixed-mode circuits, which contain both analog and digitalcircuits.

MOSFETs fabricated with SOI technology include non-fully depletedMOSFETs with silicon film thickness greater than the maximum channeldepletion width and fully-depleted MOSFETs having silicon film thicknessless than the maximum channel depletion width. Unlike bulk siliconMOSFETs, the substrate of an SOI MOSFET is usually electricallyfloating. In a non-fully depleted MOSFET, carriers (holes in nMOSFETsand electrons in pMOSFETs) generated by impact ionization accumulatenear the source/body junction of the MOS transistor, and eventuallysufficient carriers will accumulate to forward bias the body withrespect to the source thus lowering the threshold voltage through thebody-bias effect. Extra current will start flowing resulting in a "kink"in the I-V characteristics as shown in FIG. 1. This reduces theachievable gain and dynamic swing in analog circuits, and gives rise toabnormality in the transfer characteristics in digital circuits.

In a fully-depleted SOI MOSFET, the channel is depleted completely undernormal operations. The source/channel junction has a lower potentialbarrier, and the carriers generated by impact ionization have smallereffect on the body and channel potential, thus the "kink" softens. Butthe resulting output resistance as illustrated in FIG. 2 is poor, thusmaking SOI technology less attractive than conventional bulk technologyin analog circuits.

Furthermore, in fully-depleted MOSFETs, the depletion charge is reducedfor a given body doping concentration, leading to a smaller thresholdvoltage. Threshold voltage becomes very sensitive to variations in thesilicon film thickness, which makes the fabrication of high performancecircuits very difficult. Additionally, the reduction of silicon filmthickness in a fully-depleted MOSFET gives rise to high source/drainseries resistance which in turn lowers the device operation speed.Silicidation can help improve the series resistance, but it will createmechanical stress and the process is hard to control on thin filmsilicon. One solution to the series resistance problem is to selectivelyreduce the silicon film thickness over the channel region. However, theresulting recessed region and the polysilicon gate are not automaticallyaligned. To allow for the possible misalignment, the recessed thinsilicon region must be made longer than the gate. This reduces thedevice performance and density, and results in asymmetrical devices.

Another problem common to both fully-depleted and non-fully-depleted SOIMOSFETs is the parasitic floating-base lateral bipolar transistorexisting in parallel with SOI MOSFETs. Band-to-band tunneling generated(GIDL) drain leakage and drain/body junction leakage due to thermalgeneration are multiplied by the gain of the parasitic bipolar junctiontransistor, which might be as high as 100. Low breakdown voltage andanomalously steep subthreshold slope due to a decreasing thresholdvoltage has been observed.

Problems due to the floating body can be solved by providing a contactto the body for hole current collection. However, the currentlyavailable hole collection schemes, including the use of a side-contactor the use of a mosaic source are very inefficient and consumesignificant amounts of device area. A dual source structure has beenproposed which depends on an aluminum spiking phenomenon to make contactto the P region, which is sensitive to process variations. Further, thestructure is not compatible with VLSI junction and contact technologysuch as silicidation.

Another major obstacle to the use of SOI technology in production iselectrostatic discharge (ESD) susceptibility. In bulk-substratetechnology, good ESD protection levels have been demonstrated by usingnMOS/CMOS buffers. However, this protection scheme is not compatiblewith SOI structures. For example, thick-field-oxide devices are notavailable on an SOI substrate. Large-area low-series-resistance(vertical) PN junctions are not available as the silicon film can bethinner than 100 nm. Experimental results demonstrate that ESDperformance on SOI wafers are much worse than bulk technology. This canbe due to two reasons, namely the poor thermal conductivity of theburied oxide enhances the failure due to Joule heating, and thereduction of silicon film thickness and junction depth increases the ESDcurrent density. Severe localized silicon heating can result, whichcauses junction melting and polysilicon melt filaments to form, whichcause electrical shorts among the gate, source, drain and body of thetransistors and result in device failure. ESD protection schemesdesigned for SOI circuits have been proposed using additional circuitsconstructed with diodes and polysilicon resistors, however these devicesconsume large silicon area, introduce large delays, and are far fromadequate.

The present invention is directed to providing SOI transistors whichovercome or reduce the above problems.

SUMMARY OF THE INVENTION

Briefly, in accordance with the invention, a low barrier body contact isprovided under at least the source of an SOI transistor to collectcurrent generated by impact ionization, junction leakage, and tunneling.For an nMOSFET, a P-doped region is provided under the N⁻ source region.A side contact can be provided to the underlying current collectingregion, or tunneling conduction from the underlying region to theoverlying source region can accommodate the collected current.

In accordance with another feature of the invention, thickersource/drain regions are provided with the channel region therebetweenrecessed to reduce series resistance and increase operating speed of thetransistor. Transistors with different conduction voltages, V_(T), canbe accommodated by the channel thinning and by threshold dopant implantinto the channel region. Thus, fully-depleted SOI MOSFETs andnon-fully-depleted SOI MOSFETs can be integrated on the same chip. Fullydepleted SOI transistors with recessed channel structures are suitablefor digital circuits because of suppression of punch-through and thekink effect. Non-fully-depleted SOI MOSFETs with the low barrier bodycontact are ideal for analog applications, but require a thicker body.Both analog and digital transistors can thus be accommodated on the samechip giving ultimate integration for a wide range of circuitapplications. A self-aligned gate and recessed channel process isprovided in accordance with the invention.

To provide electrostatic discharge (ESD) protection, buffer transistorscan be fabricated directly in the substrate adjacent to an SOItransistor by first removing the semiconductor film and buried oxidelayer. The fundamental problem of thermal isolation due to the buriedoxide is thus eliminated. In an alternative embodiment using an SOItransistor for ESD protection, the transistor is fabricated inelectrically isolated islands or in physically isolated mesas, so thateach island or mesa can have at least one conducting filament andabsorbs some amount of ESD energy. The total ESD energy is thus spreadamong the isolated regions, whereas in a bulk SOI transistor the ESDcurrent tents to concentrate at a few hot filaments which can lead todevice failure.

The invention and objects and features thereof will be more fullyunderstood from the following detailed description and appended claimswhen taken with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of I-V characteristics for a non-fully-depleted SOIdevice.

FIG. 2 is a plot of I-V characteristics for a fully-depleted SOI device.

FIG. 3 is a section view of an SOI MOSFET in accordance with oneembodiment of the invention.

FIG. 4 is a graph illustrating a comparison of performance betweenconventional and SOI MOSFETs in accordance with the structure of FIG. 3.

FIG. 5 is a section view of another embodiment of an SOI MOSFET inaccordance with the invention.

FIGS. 6A-6F are section views illustrating the fabrication of aself-aligned recessed channel SOI MOSFET in accordance with theinvention.

FIG. 7 is a section view of an SOI MOSFET and a bulk MOSFET for ESDprotection in accordance with the invention.

FIGS. 8 and 9 are graphs illustrating ESD failure voltage of nMOSFETshaving different L_(eff) under positive ESD stress.

FIG. 10 is a plot of ESD failure voltage as a function of channel width.

FIGS. 11 and 12 are plan views of multiple island ESD production devicesin accordance with the invention.

FIGS. 13 and 14 are plan views of conventional SOI ESD protectiondevices comprising a single silicon island.

FIG. 15 is a section view illustrating the fabrication of resistors inthe SOI ESD multiple island devices of FIGS. 11 and 12 fabricated inlightly-doped polysilicon films using a light-doped drain implant orsource/drain diffusion.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Features of the invention will be described with reference to specificnMOSFET embodiments, but it will be appreciated that the inventionapplies to pMOSFETs as well.

Referring to FIG. 3, an nMOSFET having a low barrier body contact sourceregion in accordance with the invention is illustrated in cross-section.As in a typical SIMOX structure, a silicon substrate 10 has a buriedsilicon oxide layer 12 therein with the transistor fabricated in anoverlying monocrystalline silicon film. The transistor includes an N⁺doped drain region 14 and an N⁺ doped source region 16 formed over a Pdoped body contact 18. P⁺ region 20 abutting the source 16 and bodycontact 18 provides a current path for the impact ionization currentaccumulated in region 18 near the source/body junction. The sourceregion 16 and drain region 14 are separated by a channel region 22 witha polysilicon gate contact 24 overlying the channel region 22 andinsulated therefrom by silicon oxide. Metal or polysilicon interconnectlines 26, 28 are interconnected with the source and drain, respectively,with the P⁺ region 20 also connected to the source contact line 26.

The P region 18 underlying the source region, either neutral ordepleted, provides a low resistance path to collected hole currentgenerated by impact ionization, junction leakage, and GIDL. The Pimplant for the underlying P region 18 is on the order of 1×10¹⁴ to1×10¹⁶ dopant atoms per cm⁻². The N+ implant for the source and drain ishigher in dopant concentration with a lower implant energy level to formthe shallower N+ doped regions. The P- doped low barrier contact can beprovided under both the source and drain for a simpler fabricationprocess. The P+ contact 20 is formed after the N+ implant.

FIG. 4 is a plot illustrating a comparison of performance betweenconventional SOI MOSFETs and MOSFETs with the low barrier body contact,as illustrated in FIG. 3. The kink effect in non-fully-depleted SOIMOSFETs is completely removed and the bipolar induced problems aresuppressed. The low barrier body contact MOSFETs exhibit higherbreakdown voltage, especially for low gate voltages, and a very constantsaturated drain current which is free of the kink. The resulting devicehas all of the advantages of a bulk MOSFET in addition to the bettershort channel behavior and lower capacitance advantages of SOIstructures, which is ideal for analog circuits.

The low barrier body contact structure in accordance with the inventionis capable of collecting carriers uniformly across the channel with amuch higher efficiency than the conventional side substrate contact orthe mosaic source while requiring less surface area than theconventional schemes. Some transistors in a circuit can be designed tohave the low barrier body contact structures while others have floatingbodies in order to perform pass-gate functions or have the largercurrent of a floating body MOSFET. Even without the current collection Pregion (N region for P MOSFETs) beneath the source, the carriers canstill be collected by the source if the doping concentration at thebottom of the source is sufficiently low to provide a low barrier forcollecting the impact ionization generated carriers.

FIG. 5 is a section view of another embodiment of the invention in whichthe P⁺ layer is placed beneath the N⁺ source, and contact is made onlyto the N⁺ source. Like elements in FIGS. 3 and 5 have the same referencenumerals. Instead of relying on the non-uniform, therefore uncontrolled,spiking of contact, the electrical current between the N⁺ and P⁺ regionsis carried out by uniform tunneling conduction between the N⁺ and P⁺silicon as is the well-known mechanism of the operation of a tunneldiode. Even though the N⁺ regions at the source and drain are ofdifferent depths, they can be created by the same N⁺ implantation.

In the embodiments of FIGS. 3 and 5, the low barrier body contact isprovided only under the source region of the SOI MOSFET. However, asimilar low barrier body contact can be provided under the drain regionthereby providing a symmetrical transistor structure which is desirablefor applications such as a pass-gate circuit.

In accordance with another feature of the invention, a thicker film ofsilicon is employed for the source and drain regions to reduce seriessource/drain resistance and increase operation speed. Further,non-fully-depleted SOI MOSFETs require a thicker body. However, thethicker source and drain regions require a thinning of the channelregion therebetween. FIGS. 6A-6F are section views illustrating a methodof fabricating a self-aligned gate and recessed channel structure inaccordance with a feature of the invention. In FIG. 6A a substrate (notshown) has a barrier oxide 40 therein with a thicker silicon film 42thereon. A thin layer of silicon oxide 44 is grown on the surface of thesilicon film and a layer of silicon nitride 46 is deposited on the oxidelayer. A window 48 is then etched through the nitride and oxide layers.Thereafter, a sacrificial LOCOS oxide 50 is grown through window 48 intothe silicon layer 42 to thin the silicon film where the MOSFET channelwill be. An oxide etch is then employed using the nitride layer 46 as amask to expose the channel region of the silicon film, as shown in FIG.6C. In FIG. 6D, gate oxide is grown on the exposed surface of thesilicon film and a polysilicon layer 52 is then formed in the etchedhole and over the surface of the silicon nitride. In FIG. 6E, thepolysilicon film and nitride are removed, leaving the polysilicon gateelectrode 53 aligned over the channel region and separated therefrom bythe gate oxide from FIG. 6D. Finally, in FIG. 6F, dopant ions areimplanted in the source and drain regions using the gate electrode forself-alignment, oxide 56 is deposited and selectively etched, and metalsource and drain interconnect lines 57 and 58 interconnect thetransistor structure. In addition to the thicker source/drain regionswhich can be provided, ultrathin film SOI transistors down to 10 nm canbe fabricated using this technique without a problem of the source anddrain regions being consumed during contact opening and metallization.

Circuit operating voltage must be reduced in order to reduce powerconsumption. When the operating voltage is lowered, the MOSFET thresholdvoltage (V_(T)) must be lowered in order to obtain larger currents andhigher circuit speed. Unfortunately, a low V_(T) leads to largetransistor leakage, which increases power consumption. By using the lowbarrier body contact and recessed channel structure of the invention,MOSFETs in a speed-sensitive and leakage-insensitive portion of thecircuit can have lower V_(T) while other MOSFETs have a higher V_(T)through use of dual silicon film thickness produced in the recessedchannel structure. Differences in V_(T) can be controlled by performingthreshold adjust implants before or after the LOCOS thinning step (FIG.6B) and by adjusting the amount of additional uniform thinning after theLOCOS thinning step. Different V_(T) for each of P and N channeltransistors can be obtained by masking the V_(T) implant for sometransistors with a photoresist mask and using the dual silicon filmthickness. Two additional V_(T) can be obtained by implanting P channelthreshold adjust ions into N channel transistors, and vice versa forpMOSFETs. The use of different silicon film thicknesses on the same chipmakes device design very flexible. Fully-depleted SOI MOSFETs andnon-fully-depleted SOI MOSFETs can be integrated on the same chip withthe fully-SOI transistors with a recessed channel structure beingparticularly useful for digital circuits because of suppression ofpunch-through and the kink effect. Non-fully-depleted SOI MOSFETs with alow barrier body contact are ideal for analog applications but require athicker body, all of which being accommodated on the same chip inaccordance with the invention to give ultimate integration covering awide range of applications.

In accordance with another aspect of the invention, electrostaticdischarge (ESD) protection can be incorporated in SOI MOSFET circuits.In accordance with one embodiment shown in cross-section in FIG. 7, anSOI MOSFET 64 can be fabricated on the oxide layer 68 of substrate 70,while adjacent to transistor 64 a bulk MOSFET 66 is fabricated directlyin substrate 70 by etching through the silicon film and buried oxide tocreate an opening to expose the silicon substrate in which thetransistor 66 is fabricated. ESD protection circuits including nMOSbuffers, pMOS buffers or CMOS buffers can then be fabricated on thesilicon substrate at these openings. Accordingly, most of the bulk ESDprotection circuits are directly transferred to the SOI technology, thussaving time and effort of developing special circuits for SOI ESDprotection. The fundamental problem of thermal isolation due to theburied oxide in SOI technology is eliminated, hence giving moreflexibility for building the ESD protection circuits. Experimentalresults demonstrate that ESD protection circuits built directly in thesubstrate are capable of withstanding higher ESD discharge voltageduring the human-body-model stress compared with ESD protection circuitsbuild on conventional SOI circuits for both positive and negativedischarge pulses. For negative discharges, conventional SOI NMOStransistors in the buffer are operated in the so-called transistor-diodemode. The series resistance in this operating mode is too large for SOItechnology to provide satisfactory ESD protection. However, with bulkMOSFETS, the negative discharge pulses are absorbed by the largedrain-to-substrate diode which can accommodate larger ESD currents. Theresults are summarized in the plots of FIGS. 8 and 9 which show ESDfailure voltage of different nMOSFETs with different L_(eff) underpositive ESD stress. The transistors have W_(eff) of 250 μm, T_(ox) of 9nm, and 2 μm contact-to-gate spacing.

In accordance with another feature of the invention, an ESD protectiondevice can be built into the silicon film in thermally isolated segmentsto increase the ESD current handing capability. FIG. 10 is a plotillustrating a special problem of SOI ESD protection, namely, SOI ESDfailure voltage does not increase significantly with increasingprotection device width. This prevents the attainment of good ESDprotection by simply enlarging the device width of an SOI transistor asis routinely done for bulk IC ESD protection. The problem lies in thefilamentary current flow in the ESD devices under the condition ofsecondary breakdown. Current is concentrated at hot filaments while coldregions of a transistor carry no current. In a wide bulk transistor,there are many hot spots (filaments) and each filament can absorbcertain ESD current or energy without failure. However, in an SOIdevice, it is unlikely to have multiple hot spots since the temperaturein a silicon island tends to be more uniform even in a wide transistorbecause the thermally-conductive silicon island is surrounded bythermally insulating oxide. In accordance with the invention, the SOIESD protection device is divided into a plurality of mesas or in siliconislands, each separated by oxide, as illustrated in the plan views ofFIGS. 11, 12. Each island can have at least one conducting filament andabsorb a certain amount of ESD energy so that the 250 μm-wide SOI devicewould have about 5 times larger ESD voltage as a 50 μm-wide device,rather than a similar ESD failure voltage as shown in FIG. 10. The useof multiple islands requires little more area than conventional SOIprotection devices shown in FIGS. 13 and 14. The silicon islandsillustrated in FIGS. 11, 12 can be created by field oxide isolation orby physical isolation through etching of mesas. The resistors in thesubject of FIGS. 11 and 12 can comprise part of a lightly doped drain(LDD) structure as illustrated in FIG. 15 and are added to promoteuniform distribution of the ESD current.

SOI MOSFETs in accordance with the invention improve operatingcharacteristics and speed through use of the low barrier body contact,and the inclusion of non-fully-depleted SOI MOSFETs along withfully-depleted SOI MOSFETs in one SOI integrated circuit can beaccommodated using the recessed channel structures along with multiplethreshold voltage implants. Further, improved ESD for protection for SOIcircuits is readily provided by forming bulk transistors directly in theunderlying substrate and through the use of device segmented islands ormesas.

While the invention has been described with reference to specificembodiments, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A method of fabricating a field effect transistorin a silicon layer over an insulating layer comprising the steps of(a)forming a silicon oxide layer on said silicon layer, (b) forming asilicon nitride layer on said silicon oxide layer, (c) removing aportion of said silicon oxide layer and said silicon nitride layer toexpose a portion of said silicon layer, (d) oxidizing said portion ofsaid silicon layer thereby forming a thick oxide layer and therebythinning said silicon layer to form a recessed portion, (e) removing aportion of said thick oxide to expose said recessed portion, (f) growinga gate oxide on said recessed portion, (g) depositing polysilicon onsaid gate oxide and over said silicon nitride layer, (h) removing bothsaid polysilicon overlying said silicon nitride layer and said siliconnitride layer thereby leaving a polysilicon gate electrode on said gateoxide, and (i) forming source and drain regions in said silicon layerusing said polysilicon gate electrode as a dopant mask forself-alignment.
 2. A method of fabricating a field effect transistor ina silicon layer over an insulating layer comprising the steps of(a)forming a silicon oxide layer on said silicon layer, (b) forming asilicon nitride layer on said silicon oxide layer, (c) removing aportion of said silicon nitride layer over a portion of said siliconlayer, (d) oxidizing said portion of said silicon layer thereby forminga thick oxide layer and thereby thinning said silicon layer to form arecessed portion, (e) removing a portion of said thick oxide to exposesaid recessed portion, (f) growing a gate oxide on said recessedportion, (g) depositing polysilicon on said gate oxide, and (h) removingboth said polysilicon overlying said silicon nitride layer and saidsilicon nitride layer thereby leaving a polysilicon gate electrode onsaid gate oxide.
 3. The method as defined by claim 1 and furtherincluding the step of implanting dopant into a channel region under thepolysilcon gate electrode.
 4. The method as defined by claim 2 andfurther including the step of implanting dopant into a channel regionunder the polysilcon gate electrode.